1. Field of the Invention
The present invention relates, generally, to injection molding systems, and more particularly, but not exclusively, the invention relates to hot tips of nozzles for hot-runners, particularly nozzles where a tip insert is secured by a separate retainer piece.
2. Background Information
The state of the art includes various nozzles and tips for hot runner injection molding systems. Hot-runner nozzles are typically either a valve-gate style or a hot-tip style. In the valve-gate style, a separate stem moves inside the nozzle and tip acting as a valve to selectively start and stop the flow of resin through the nozzle. In the hot-tip style, a small gate area at the end of the tip freezes off to thereby stop the flow of resin through the nozzle. The present invention applies to the hot-tip style nozzles.
An injection molding system using a hot-tip style nozzle typically has a cooled mold with a small circular gate opening in which the tip of the nozzle is inserted. The tip is typically conical with a tapered point. In operation, the tapered point is positioned in the circular gate to thereby form an annular opening through which molten plastic is then transferred from the nozzle to the mold. When the mold is full, plastic flow stops. In an ideal molding cycle, the mold is typically cooled so that the plastic injected into it soon solidifies. As the liquid plastic in the mold cools it shrinks, which continues to allow plastic from the nozzle to move into the mold. This step is referred to as xe2x80x9cpackingxe2x80x9d. The nozzle is typically heated so that the molten plastic contained within it remains liquid. The hot plastic moving through the gate area during packing keeps the gate area from freezing until all the plastic in the part has solidified. Eventually the gate freezes, the mold is opened, and the part is ejected, thereby breaking the small amount of frozen plastic at the gate area.
If the mold is opened before the gate has frozen, the plastic will string from the nozzle to the mold. This is known as a xe2x80x9cgate stringingxe2x80x9d and is unacceptable because the plastic string must be removed from the part in a subsequent operation, or the part scrapped. Waiting a long time for the gate to freeze is also unacceptable because it adds time to the molding cycle, which is desired to be as short as possible to optimize system productivity.
If the nozzle does not provide enough heat at the tip to keep the gate from freezing before the part is fully injected and packed, the part may have voids or other quality problems making it unacceptable.
In the prior art, direct heating of the nozzle tip is not practical because of the small size of the nozzle tip. Consequently, the nozzle tip must be heated through a conduction process reliant upon the conductivity of the materials in the entire nozzle. Heat is applied to the nozzle body by well-known techniques and is conducted to the nozzle tip. The tip material is, therefore, made of high-conductivity material that promotes the flow of heat to the nozzle tip.
It is important that the nozzle tip provide the right amount of heat at the gate area to keep the plastic in a liquid state as it flows through the gate, but also that it allows the plastic to freeze in a reasonable time when flow has stopped. The tip must also resist corrosion, sustain compressive loads from injection pressures that may reach 40,000 psi, (275 Mpa) or higher, and resist wear when used with plastics containing fillers such as glass or other particulate materials. Since tips can wear out, it is desirable that they be easily replaceable. It is also desirable that tips be easily changed to process different materials.
To address those needs, two-piece tip assemblies have become commonplace. A removable tip insert is secured in the nozzle housing by a tip retainer that typically threadably engages the nozzle housing. The relatively inexpensive tip insert can easily be changed by unscrewing the retainer, installing another tip insert, and reattaching the retainer. Such tip arrangements are cost-effective because the retainer is not replaced.
The two-piece tip assemblies include a nozzle seal that is attached to the retainer portion. Since the nozzle seal contacts the mold, which is cold relative to the nozzle tip, it is preferable that the nozzle seal material has low thermal conductivity so that heat from the nozzle and nozzle tip is not transmitted into the mold through the nozzle seal.
These requirements have resulted in several US patents on a variety of tip and nozzle seal arrangements that use a tip insert and a retainer, and they all teach the use of a highly thermally conductive material for the tip insert.
U.S. Pat. No. 5,208,052 to Schmidt et al. teaches a tip insert made from beryllium copper, having a high thermal conductivity, and a retainer made from titanium alloy having low thermal conductivity.
U.S. Pat. No. 5,299,928 to Gellert also teaches an inner piece of the tip formed of a highly thermally conductive material, such as beryllium copper alloy, and the outer retaining piece formed of a material such as titanium alloy which is much less thermally conductive than the beryllium copper tip insert.
Likewise, U.S. Pat. No. 5,885,628 to Swenson et al. teaches an inner piece of the tip constructed of a highly conductive material, such as beryllium copper, and an outer piece preferably formed of a low thermally conductive material, such as titanium alloy.
U.S. Pat. No. 6,394,785 to Ciccone also discloses a nozzle tip insert normally made of beryllium copper.
For more wear-resistant tips, U.S. Pat. No. 6,302,680 to Gellert et al. discloses a tip insert made of a material, such as beryllium copper or tungsten carbide copper, having a combination of thermal conductivity and wear and corrosion resistance suitable for the material being molded. The nozzle seal, which also retains the tip insert, is made of suitable wear and corrosion resistant material, such as stainless or H-13 tool steel. U.S. Pat. No. 6,164,954 to Mortazavi et al. also discloses the use of materials for the tip insert that exhibit high wear resistance and good thermal conductivity, such as carbide and tungsten carbide. Mortazavi also discloses the use of materials for the retainer that have good thermal conductivity, such as Ti/Zr-carbide.
U.S. Pat. No. 5,879,727 to Puri discloses a nozzle tip preferably made of a material with a relatively high thermal conductivity, such as copper-based alloys. The tip threadably attaches to the nozzle, and a seal ring, made of relatively high wear resistant material such as H-13, 4140 or P-20 tooling metals, attaches to the tip it through an insulator made of a low thermally conductive material such as titanium,
All of these nozzle tips function in essentially the same way, using the high thermal conductivity of the tip insert to conduct heat from the heated nozzle body to the gate area. The heat from the nozzle tip opens the gate at the beginning of the injection cycle and keeps it open during the injection process, and cooling from the mold cools and eventually freezes the gate after packing is complete. If the tip is not hot enough, the gate may not open and injection will not occur, or the gate will freeze too soon causing poor-quality parts. If too much heat is transferred to the tip, the gate will not freeze, resulting in stringing gates. Therefore, for any particular nozzle tip and resin there is an operating temperature window between the minimum temperature needed to get the gate open and keep it open as desired through the molding process, and the maximum temperature at which parts can be made without stringing gates. If the operating window is narrow, it may be difficult for molds with multiple cavities to consistently make good parts in all cavities because of the many variables associated with the injection molding process. One factor is assembly tolerance stack up that varies tip heights in the gate. Since the tip is conical, variations in tip height cause variations in the size of the annulus between the tip and the gate through which molten plastic flows. Another factor is variation in temperature of the resin from the nozzle to nozzle due to heat loss at various portions in the hot runner, or from flow imbalance in the hot runner.
Furthermore, resins have melt flow characteristics and an optimum temperature range for processing that determines what processing parameters are used in the injection molding process. The flow characteristic for a resin inherently varies from batch to batch. To keep resin costs down and to preclude sorting resin by batch, molders often purchase resins in large quantities with a specification allowing a large range for flow characteristic. One batch of resin may run adequately for a given set of processing parameters, but the next batch, having a different flow characteristic, may not produce good parts using exactly the same process settings.
In many injection molding systems, the temperature of the nozzle is often monitored, typically near the tip, with a thermocouple. This measures the temperature of the metal in the nozzle, and not the actual temperature of the molten resin. There is a loop feedback arrangement between the thermocouple and the nozzle heater to typically keep the nozzle temperature, as measured by the thermocouple, at a fixed setpoint. On some systems, the temperature of each individual nozzle can be adjusted as necessary to compensate for the above variations, but on many systems such individual control of the nozzle heaters is not possible. Still on other systems, the nozzle temperature is not monitored. Based on process experiments, these systems rely on power control settings, typically some percentage of maximum available power, to determine the operating temperature of the nozzle. With any system, however, it is unfortunately possible to have one nozzle operating at the upper limit of the operating window, and another nozzle operating at the lower limit. Or, if the window is too narrow, some nozzles may always be outside of the window, thereby preventing the mold from producing good parts in all cavities.
There is a need for an injection molding nozzle tip which provides a wider operating window than those of the prior art.
The present invention provides a nozzle for an injection molding runner system. The nozzle comprises a nozzle housing having a melt channel through it, a nozzle tip having a tip channel and at least one outlet aperture in communication with the tip channel, and a tip retainer that retains the nozzle tip against the nozzle housing such that the tip channel communicates with the melt channel. The tip retainer is significantly more thermally conductive than the nozzle tip.
In one embodiment, the nozzle includes a nozzle seal that is significantly less thermally conductive than the tip retainer. The nozzle seal may be fused with the tip retainer, and may be annularly spaced from the nozzle tip.
In one embodiment, the tip retainer is removably affixed to the nozzle housing by threads. In another embodiment, the tip retainer is removably affixed to the nozzle housing by low-temperature brazing. In yet another embodiment, the nozzle tip is retained in the tip retainer by low-temperature brazing.
Advantageously, the present invention provides a nozzle tip having a large operating window that permits the production of high quality parts even in the face of large variations in resin temperature, tip height and resin flow characteristic. The nozzle tip of the preferred embodiment of the present invention also beneficially avoids having to adjust nozzle temperatures.